Electron injection plasma variable reactance device with perforated anode in the electron path



April 15, 1969 I 3,439,225

ELECTRON INJECTION PLASMA VARIABLE REAC'IANCE DEVICE WITH PERFORATEDANODE IN'THE ELECTRON PATH Filed Oct. 24, 1966 Sheet of 3 April 15, 1969KNECHTL' 3,439,225 ELECTRON INJECTION PLASMA VARIABLE REACI'ANCE DEVICEWITH PERFORATED ANODE IN THE ELECTRON PATH Filed Oct. 24, 1966 Sheet 3Of 3 I I I 9/ a (5 I T 57481.5 W727 774/. sauna-5 .aza

April 15, 1969 c- K E I 3,439,225

ELECTRON INJECTION PLASMA VARIABLE REACTANCE DEVICE WITH PERFORATEDANODE IN THE ELECTRON PATH Filed Oct. 24, 1966 Sheet 5 Dr s Para/rm;

United States Patent Office 3,439,225 Patented Apr. 15, 1969 3,439,225ELECTRON INJECTION PLASMA VARIABLE RE- ACTANCE DEVICE WITH PERFORATEDANODE IN THE ELECTRON PATH Ronald C. Knechtli, Woodland Hills, Calif.,assignor to Hughes Aircraft Company, Culver City, Calif., a corporationof Delaware Filed Oct. 24, 1966, Ser. No. 589,119 Int. Cl. HOlj 19/80,7/46; H01p 1/00 US. Cl. 315-39 10 Claims ABSTRACT OF THE DISCLOSURE Theelectron injection plasma device injects electrons into a gas in awaveguide so that plasma is created, with the plasma density determiningthe reactance of the plasma with respect to radio frequencyelectromagnetic energy transmitted through the waveguide. The deviceincludes a perforated anode arranged so that the electrons are injectedthrough the perforated anode, reflect off the far wall of the waveguideand return to the anode.

This invention relates to a variable reactance device and moreparticularly to a plasma variable reactance device providingelectronically controllable variable reactance to radio frequencyelectromagnetic energy.

The use of variable reactance devices as either fixed or controllablereactance devices is well established in the art. For example, variablereactance devices have been used for electronic control of antennaarrays, especially slot antenna arrays; electronic switching of moderateto high power microwave energy; frequency multiplication; parametricamplification; electronically controlled phase shifters, to name just afew.

Variable reactance devices are generally classified into threecategories-solid-state or semiconductor varactors, ferrite variablereactance devices and plasma variable reactance devices.

For low radio frequency (RF) power levels, solid-state variablereactance devices provide useful electronically controlled RFelectromagnetic energy reactances, especially at microwave frequencies.Typical characteristics of a good solid-state variable reactance devicesare a quality factor or Q of the order of 10 at X-band, but a microwavepower handling capability of the order of only one watt.

Ferrite devices have been commonly used for electronic switching and/orphase control of microwaves but are somewhat limited in the amount ofaverage RF power they can handle. Also, these devices introducesignificant RF losses and are temperature sensitive to a substantialdegree.

Variable reactance devices utilizing the plasma of a gas discharge arecalled plasma variable reactance devices and also are capable ofproviding an electronically controllable variable reactance. This typeof variable reactance device is generally less efficient thansolid-state variable reactance devices, introduce a considerable amountof noise to the system and have a relatively low quality factor or Q.

The electron injection plasma variable reactance devices of theinvention on the other hand have the advantage of being able toeffectively operate at relatively high average RF power levels andintroduce lower RF losses than ferrite devices, for example, and alsoare less temperature sensitive. Variable reactance devices constructedaccording to the invention furthermore have a relatively lowerequivalent noise temperature, a much higher Q and lower insertion lossthan conventional plasma variable reactance devices.

Accordingly, it is an object of this invention to provide an improvedelectronically controllable radio frequency electromagnetic energyreactance device.

It is another object of the invention to provide a more eflicient plasmavariable reactance device than heretofore obtainable.

It is still another object of the invention to provide a plasma variablereactance device wherein the plasma is maintained substantially freefrom unwanted oscillations and has a low equivalent radio frequencynoise temperature.

It is yet another object of this invention to provide a variablereactance device with a very low RF insertion loss.

It is a further object of the invention to provide a variable reactancedevice having a relatively high Q.

It is still a further object of the invention to provide a variablereactance device having a high RF power handling capability. The abovementioned and other objects of the invention are achieved in an electroninjection plasma variable reactance device adapted to interact withradio frequency electromagnetic energy. According to one embodiment ofthis invention, the variable reactance device comprises an electronemitter (cathode), an electron repeller, a gridanode element disposedbetween the emitter and repeller, and a gaseous medium maintained in.the region between the emitter and repeller. The emitter, repeller andgridanode elements are connected to an appropriate adjustable potentialsource to create a plasma sheath having an injection boundary from whichelectrons emitted by the emitter are injected into the gas and. repelledby the repeller and collected by the grid-anode element. The injectedelectrons travel a mean distance L from the injection boundary to thecollector surface whereby the gas is ionized to form a plasma of adensity dependent upon the magnitude of the potential difference betweenthe plasma sheath and the collector and upon the discharge current. Thegas is maintained at a pressure such that the ionization mean free pathfor the injected electrons is at least of the order of the distance L.Also, the device is arranged so that there is an interaction between theradio frequency electromagnetic energy to be influenced and the plasma.

The invention and specific embodiments thereof will be describedhereinafter by way of example and with reference to the accompanyingdrawings wherein like reference numerals refer to like elements andparts, and in which:

FIG. 1 is a schematic diagram and associated potential distributioncurve of a reflex electron injection plasma triode variable reactancedevice according to the invention;

FIG. 2 illustrates a reflex variable reactance device of the type shownin FIG. 1 that is integrated in a waveguide;

FIG. 3 is a schematic diagram and associated potential distributioncurve of a reflex electron injection plasma tetrode variable reactancedevice according to the invention;

FIG. 4 is a cross sectional view of a tetrode reflex variable reactancedevice of the type shown in FIG. 3 integrated in a waveguide; and

FIG. 5 is a cross-sectional view of a plasma variable reactance devicehaving a tetrode configuration.

In order to facilitate in the description of the invention, thefollowing material relating to plasma variable reactance devices ingeneral should be noted.

A plasma variable reactance device is a device producing a plasma ofcon-trolled density over an appropriate volume. The plasma acts as adielectric Whose dielectric constant e is determined by the electrondensity n of the plasma and is given by Heald & Wharton at p. 6 inwhere:

n Electron density m =Electron mass e=Electrn charge 'y Electroncollision frequency w=21rf (RF field frequency) e =Dielectric constantof free space It is seen that an increase in electron density n resultsin a reduction of dielectric constant e, which means essentially thatthe plasma behaves as an inductive medium, the inductive effect of theplasma being controlled by controlling n As an example, at X-band (f=c.p.s.), in order to make 6 0, it can be shown that a plasma density n=l0 electrons/0111. will be required. This value can readily be obtainedby means of gas discharges.

The RF losses of a plasma variable reactance device can be calculated bymeans of Equation 1 and are best expressed in terms of the Q of thereactance presented by the plasma to the RF fields. Q is defined as:

where W=Kinetic RF energy stored in plasma P=RF power dissipated inplasma.

Power dissipation in the plasma is caused by electronion andelectron-neutral collisions, which appear through the factor 7 inEquation 1. To evaluate Q from Equations 1 and 2, it may be assumed tothe first approximation that the plasma is uniform and occupies a volumeV .(constant dielectric constant 6 within the volume V). Then:

where:

E=RMS RF electric field.

From the above relationship and Equation 1:

Equation 3 shows that in order to make a high Q (low RF loss) plasmavariable reactance device, it is necessary to use a gas discharge inwhich the collision frequency v is low. In the type of discharge devicesto be considered, electron-neutral collisions predominate overelectron-ion collisions and therefore the problem is to minimize theelectron-neutral collision frequency. To obtain this goal, it isnecessary to minimize the gas pressure for a given electron density n Ina conventional positive column discharge, this can only be done at thecost of a prohibitive discharge sustaining power. The practical limitfor the Q of a plasma variable reactance device using a positive columndischarge has been found to be of the order of 3 at X-band, which is toolow to be of practical interest. The following described deviceutilizing the principle of electron inject-ion according to theinvention operates at a lower gas pressure for a given electron densityn and for a given discharge power than positive column and otherconventional discharge devices.

The gas discharge to be used in electron injection plasma variablereactance devices is characterized by the injection of electrons intothe gas to be ionized, at an energy substantially higher than theionization potential of the gas. This electron injection takes placeaccording to this invention through a cathode sheath as can be seen fromthe potential distribution curve of FIG. 1. The electron injectionenergy is approximately equal to the applied discharge voltage and cantherefore be externally controlled. By adjusting the electron injectionenergy to a value equal to or larger than about 1.5 times the ionizationpotential of the gas used, the following advantages are obtainable: (l)the ionization cross section is close to its maximum value; this permitsoperation at relatively low gas pressures, much lower than conventionaldischarges; and (2) most of the energy imparted to the electrons iseffectively used for ionization; this results in a high ionizationetficiency. Operation at low gas pressure means low electronneutralcollision frequency and high Q. High ionization efliciency means lowdischarge power.

An electron injection plasma variable reactance device constructedaccording to the invention and integrated in a waveguide is illustratedin FIG. 2. Here, the inner surface 19 of a waveguide structure 21 alsoacts as a reflector electrode to which electrons injected by an emittingsurface 23 and a cathode 25 are reflected. Supported adjacent theemitting surface 23 by a support structure 27 is an anode-grid mesh 29.The support structure 27 is of a cylindrical configuration and isuniformly spaced a short distance from a flange 31 connected to thewaveguide 21 and surrounding an aperture 33 in the waveguide 21. Thespacing between and the lengths of the grid support structure 27 and theflange 31 are chosen to act as a microwave choke at the frequency of theelectromagnetic energy propagating through the waveguide 21.

The eathode 25 is shown as an indirectly heated type utilizing a heaterelement shown schematically as heater element 35. In order to emitelectrons to form a plasma column 37, the heater element 35 is connectedby suitable means to a source of heater current, not shown. Otherpotentials, as will be described, are provided by appropriate leadsconnected to potential sources, not shown.

In this invention, the reflector surface 19 is held at a potential equalto or slightly less than the cathode potential, for best effectiveness.As shown in FIG. 2, an axial magnetic field H is provided by anyconvenient means such as permanent magnets, for example. This magneticfield is adjusted to a value such that the electron cyclotron radius forelectrons at an energy corresponding to the discharge voltage be betweenapproximately half the grid opening of the mesh of the grid 29, and theradius of the plasma column 37. For a discharge voltage (V between 10and 25 volts, a grid opening of 1 mm., and a plasma column diameter of 3mm., a magnetic field of the order of 30 to 300 Oersted should beprovided. Fields much lower than the minimum magnetic field definedabove will fail to radially confine the plasma column; fieldssubstantially higher than the maximum magnetic field defined above willlead to radial potential gradients, plasma instabilities and high RFnoise.

The advantage of the reflex discharge configuration is an increase inthe mean distance travelled by an electron before being collected by theanode-grid 29. This distance becomes several times the distance betweenthe anode-grid and the reflector. This permits a corresponding reductionin gas pressure without loss in ionization efliciency. The result is areduction in electron-neutral collision frequency approximatelyproportional to the pressure reduction. Reducing electron collisionfrequency means a reduced RF loss, i.e., increased quality factor Q.

The reactance of the invention of this reflex discharge type plasmavariable reactance device can be controlled by controlling either thereflector voltage, or the anodegrid voltage.

In the triode configuration described above, electron injection at thedischarge voltage takes place at the cathode, as indicated in FIG. 1.This implies operation close to temperature limited cathode emission(saturation). Space charge limited operation substantially below cathodesaturation current density is possible by modifying the triodeconfiguration of FIG. 1 into a tetrode configuration by addition of asecond grid between the anodegrid and the cathode. This leads to theconfiguration of FIGS. 3 and 4.

In this embodiment, as may be the case in any of the embodiments of thisinvention, the waveguide section such as waveguide 71, here, comprisespermanent magnet side walls 73 and a pole piece 75 acting as a repellerelectrode and a pole piece 77 opposite pole piece 75 and in which isdisposed a sealed plug assembly 79 having pins 81 insulatively mountedon an insulated member 83.

As described in connection with the embodiment of FIG. 2, the use of anaxial magnetic field H as provided by pole pieces 75 and 77 helps tolocalize the plasma in a well-defined column such as column 85 in orderto reduce the required discharge power for a given plasma density n Anindirectly heated cathode 87 is shown supported by wire leads connectedto two of the pins 81 which also supply heater current to the filamentelement within the cathode 87. In order to stabilize and support thecathode 87 in this position, glass or ceramic insulators 89 areconnected to an outer grid shell 91 mechanically but insulativelyconnected to the cathode 87 through an annulus insulator 93. The outergrid shell 91 includes a control mesh or grid area 95 through whichelectrons emitted by the cathode and later injected into the plasma mustpass.

Adjacent the control grid 95 on the opposite side thereof from thecathode 87 is mounted an anode-grid 97 that is supported in thisposition by a partition 99 attach ed to the inner walls of the sidewalls 73 by means of insulative support projections 100. The partition99, of course, includes an aperture 101 across which the grid 97 isstretched. The aperture opening is tapered on the side adjacent thecathode 87 to facilitate the positioning of the control-grid 95 close tothe anode-grid 97. The other of the pins 81 as shown in FIG. 4 areconnected to appropriate sources of potential and to the variouselements of this device in the same manner as described in FIG. 3, where(in the case of Xenon as the gas) Vgi has a negative potential veryclose to that of the cathode, where V has a positive potential ofapproximately 15 to 20 volts (variable from zero), and where V may havea slightly negative potential very close to the potential of the cath-'ode.

The spacing between the two grids is not critical but will preferably besmaller than the spacing between the anodegrid and the repeller. Thegrid mesh openings of the anode- -grid are best chosen to be equal tothe size of the openings of the control grid, or coarser if RF leakagedoes not become excessive with larger openings. To maintain gooddischarge efiiciency, it is advantageous to have the wires of both gridsapproximately registered. The control of the reactance of a tetrodedevice of the type of FIG. 4 is similar to that of a triode device, thefirst or control grid of the tetrode device having the same function asthe single grid of the triode device.

From the foregoing it should be evident that the plasma variablereactance devices described may be enclosed in a sealed-off containerthat is transparent to the electromagnetic energy into which thevariable reactance is to be introduced. This has the advantages ofhaving the gas restricted to the volume within the sealed-off containerthereby obviating the necessity of providing a waveguide structure thatis sealed to prevent the escape of gas. The

.gas container material can be ceramic, for example, which has a lowloss dielectric characteristic. Such a device is shown in FIG. fortetrode device but it applies equally to triode type reflex variablereactance devices as well. The variable reactance device 101 that isinserted into a waveguide section 103 through an aperture 105 in one ofthe walls thereof comprises a repeller 107 fitted into a recessedportion 109 in the inner wall, opposite the aperture 105, of thewaveguide 103. The anode 107 has a reduced diameter flange portion 111upon the outer circumference of which is sealed a low loss dielectricenvelope such as ceramic cylinder 113. At the other end of the cylinder113 is fitted another metallic flange 115 having a centrally locatedaperture and to which is attached by any convenient means, such aswelding, a cathode housing structure 117, extending away from therepeller 107 and adapted to fit in good electrical and mechanicalcontact, against the outer periphery of the aperture 105 in thewaveguide section 103 and effectively close this opening in the wall ofthe waveguide.

Within the cathode housing structure 157 is mounted by conventionalinsulative means an indirectly heated cathode 119 having an emittingsurface 121. The cathode 119 includes a heater element, not shown, thatis provided current through leads 123 connected to pins 125 passingthrough a sealed lead-through header 127 of an insulative material suchas ceramic and the like. The header 127 is sealed at its periphery to anannular curved surface flange 129 that is Welded or otherwise firmlyattached to the end of the housing structure 117 farthest from theflange 115 which is attached to the other end of this housing. Thecathode 119 is provided with a lead 131 that connects to pin 133 passingthrough the header 127 in order to allow for the proper potential beingplaced on this element. Also, a pin 135 is connected by soldering orspot welding techniques to a lead 137 that is attached by one of thesetechniques, for example, to the inner wall of the metallic housingstructure 117 to provide the repeller potential to the repeller 107through the waveguide 103.

In the case of the tetrode configuration as shown in FIG. 5, a metallicmesh or anode-grid 139 connected through a lead 140 to a pin 141 isstretched across an aperture 142 in an insulator ring 143 fitted to thecathode housing structure 117 adjacent the flange 115. This grid is thenthe anode-grid and is at a potential difference with respect to therepeller 107. A control grid 144 is supported in a position between thecathode emitting surface 121 and the anode-grid 139 by a supportstructure 145 that has a cylindrical configuration supportedsymmetrically with respect to the cathode 119 by insulating rings 147.Wire 149 connects the control grid support structure 145 to a pin 151that passes through the lead-through header 127 for connection to aproper potential, as described previously with respect to the embodimentshown in FIG. 4. As was the case with previous embodiments described, anaxial magnetic field may also be employed for the same reasons given.

Furthermore, it should be noted that the magnetic field shown for thepurposes of plasma confinement in FIG. 4, for example, may be adjustedto a value such that the electron cyclotron frequency correspondsapproximately to the frequency of the RF wave or fields to be affectedby the plasma varactor. Taking advantage of the cyclotron resonancepermits further reductions in discharge power, gas pressure, and RFlosses, and leads to a further increase in variable reactance device Q.The magnetic field required in this configuration, however, will begreater than that required for confinement of the plasma column only.

Still further, it should be understood that the designation of thelength L as shown in the drawings is only used as an aid to indicatebetween which points or places the injected electrons travel and is notshown to indicate an exact path. It should further be understood inviewing the figures that the plasma sheath thicknesses 6 are muchsmaller than the distance L.

From the foregoing, it will be evident that the invention provides animproved and more eflicient plasma variable reactance device having arelatively high Q, a very low RF insertion loss, a high RF powerhandling capability, and in which the plasma is maintained substantiallyfree from unwanted oscillations.

Although specific embodiments of the invention have been described indetail, other organizations of the embodiments shown may be made withinthe spirit and scope of the invention. For example, as a modification ofthe tetrode devices shown, the plasma between the two grids may be usedas the electronically controlled reactance instead of the plasma betweenthe second grid and the reflector.

Accordingly, it is intended that the foregoing disclosure and drawingsshall be considered only as illustrations of the principles of thisinvention and are not to be construed in a limiting sense.

What is claimed is:

1. An electron injection plasma variable reactance device for presentingan electronically controlled variable reactance to radio frequencyelectromagnetic energy, comprising:

means including an electron emitter for emitting electrons;

means including a repeller surface spaced from said emitter forrepelling the electrons emitted by said emitter;

a substantially flat perforated anode-grid structure disposed in linewith and between said emitter and repeller;

a gaseous medium maintained in the region between said emitter and saidrepeller;

means connected to respective ones of said emitter,

anode-grid, and repeller for connection to an adjustable source ofpotential to create a plasma sheath having an injection boundary forinjecting said electrons into said gaseous medium, said electronstraveling a mean distance L from said injection boundary, through saidanode-grid, toward and away from said repeller and back to saidanode-grid, said gaseous medium being ionized by said electrons injectedby said plasma sheath to form a plasma of a density dependent upon themagnitude of the potential difference between said plasma sheath andsaid anodegrid and upon the discharge current, said gaseous mediumhaving a pressure such that the ionization mean free path for saidinjected electrons is at least of the order of the distance L; and

means associated with the plasma for producing an interaction betweensaid radio frequency electromag netic energy and said plasma.

2. An electron injection plasma variable reactance device for presentingan electronically controlled variable reactance to radio frequencyelectromagnetic energy, comprising:

means including a cathode surface for emitting electrons;

means including a repeller surface spaced from said cathode forrepelling the electrons emitted by said cathode;

a substantially flat perforated anode-grid structure dis posed in linebetween said cathode and repeller;

a gaseous medium maintained in the region between said cathode and saidrepeller;

means connected to respective ones of said cathode,

anode-grid, and repeller for connection to an adjustable source ofpotential to create a cathode sheath having an injection boundaryadjacent said cathode for injecting said electrons into said gaseousmedium, said electrons traveling a mean distance L from said injectionboundary, through said anodegrid, toward and away from said repeller andback to said anodegrid, said gaseous medium being ionized by saidelectrons injected by said cathode sheath to form a plasma of a densitydependent upon the magnitude of the potential difference between saidcathode sheath and said anode-grid and upon the discharge current, saidgaseous medium having a pressure such that the ionization mean free pathfor said injected electrons is at least of the order of the distance L;and

means associated with the plasma for producing an interaction betweensaid radio frequency electromagnetic energy and said plasma.

3. An electron injection plasma variable reactance device for presentingan electronically controlled variable reactance to radio frequencyelectromagnetic energy, comprising means including a waveguide structureand a cathode surface disposed within said waveguide structure foremitting electrons;

means including a repeller surface disposed within said waveguidestructure, said repeller surface being spaced from said cathode forrepelling the electrons emitted by said cathode;

a substantially fiat perforated anode-grid structure disposed withinsaid waveguide in line with and between said cathode and repeller;

a gaseous medium maintained in the region between said cathode andrepeller;

means connected to respective ones of said cathode,

anode-grid and repeller for connection to an adjustable source ofpotential to create a cathode sheath having an injection boundaryadjacent said cathode for injecting said electrons into said gaseousmedium, said electrons traveling a mean distance L from said injectionboundary, through said anode-grid, toward and away from said repellerand back to said anodegrid, said gaseous medium being ionized by saidelectrons injected by said cathode sheath to form a plasma of a densitydependent upon the magnitude of the potential difference between saidcathode sheath and said anode-grid and upon the discharge current, saidgaseous medium having a pressure such that the ionization mean free pathfor said injected electrons is at least of the order of the distance L;and

means including windows disposed at the ends of said waveguide forallowing radio frequency electromagnetic energy to propagatetherethrough but restraining said gaseous medium within said waveguide.

4. An electron injection plasma variable reactance device for presentingan electronically controlled variable reactance to radio frequencyelectromagnetic energy, comprising:

means including a waveguide structure and a cathode surface disposedwithin said waveguide structure for emitting electrons;

means including a repeller surface disposed within said waveguidestructure, said repeller surface being spaced from said cathode forrepelling the electrons emitted by said cathode;

a control grid structure disposed between said cathode and repeller;

a substantially flat perforated anode-grid structure disposed in linebetween said control grid and said repeller;

a gaseous medium maintained in the region between said cathode and saidrepeller;

means connected to respective ones of said cathode,

control grid, anode-grid, and repeller for connection to an adjustablesource of potential to create a grid sheath having an injection boundaryadjacent said control grid for injecting said electrons into saidgaseous medium, said electrons traveling a mean distance L from saidinjection boundary, through said anode-grid, toward and away from saidrepeller and back to said anode-grid, said gaseous medium being ionizedby said electrons injected by said grid sheath to form a plasma of adensity dependent upon the magnitude of the potential difference betweensaid grid sheath and said anode-grid and upon the discharge current,said gaseous medium having a pressure such that the ionization mean freepath for said injected electrons is at least of the order of thedistance L; and

means including windows disposed at the ends of said waveguide forallowing radio frequency electromagnetic energy to propagatetherethrough but restraining said gaseous medium within said waveguide.

5. An electron injection plasma variable reactance device according toclaim 3, wherein said variable reactance device also comprises magneticfield means for localizing said plasma in a well-defined column betweensaid cathode and said repeller.

6. An electron injection plasma variable reactance device according toclaim 4, wherein said variable reactance device also comprises magneticfield means for localizing said plasma in a well-defined column betweensaid cathode and said repeller.

7. An electron injection plasma variable reactance device according toclaim 1, wherein said gaseous medium is Xenon.

8. An electron injection plasma variable reactance device according toclaim 1, wherein said gaseous medium is neon.

10 9. An electron injection plasma variable reactance device accordingto claim 3, wherein said gaseous medium is xenon.

10. An electron injection plasma variable reactance device according toclaim 4, wherein said gaseous medium is Xenon.

References Cited UNITED STATES PATENTS 2,813,999 11/1957 Foin 315-392,817,045 12/1957 Goldstein et all. 315-39 2,837,693 6/1958 Norton315-39 2,848,649 8/1958 Bryant 315-39 HERMAN KARL SAALBACK, PrimaryExaminer.

LOUIS ALLAHUT, Assistant Examiner.

US. Cl. X.R. 333--98, 99

